Visitors warned not to swim in Clear Lake, avoid boating in eastern arms due to blue-green algae

State water regulators have issued a new round of warnings about recreating in Clear Lake because of concerns about potentially dangerous blue-green algae blooms that could sicken people or pets, and they are advising boaters to steer clear of the eastern part of the lake.

“No swimming” signs are now posted at various points along the western shoreline urging visitors to avoid direct contact with discolored water or algal material.

In the southeastern arms of the lake, where the blue-green algae — technically cyanobacteria — is most densely concentrated, “danger” advisories warn boaters and others to stay off the water to avoid potential exposure to toxins.

In addition, officials said that fish caught in the Clearlake Oaks or lower arms of the lake should not be consumed because of potential contamination. Filleted fish caught in the main lake body can be eaten safely if they are cleaned with tap or bottled water and the guts discarded, they said.

The State Water Resources Control Board took the action in response to recent, routine test results that showed dangerously high levels of Microcystins, a class of potent liver toxin naturally produced in commonly occurring cyanobacteria.

Elevated levels of a neurotoxin called Anatoxin-a also have been detected, though only at high enough concentrations to prompt “caution” advisories.

The toxins have present in the lake since late July but have proliferated because of favorable conditions: persistently high temperatures and shrinking water levels.

Water samples off shore of Redbud and Austin parks in the city of Clearlake were particularly alarming during the last round of testing. In one case, testing revealed concentrations almost 200,000 times what’s considered “dangerous” because of how much water children swallow when they swim and play in water, said Sarah Ryan, environmental director for the Big Valley Rancheria.

About 280 households with private water systems that draw water from the arms in Clear Lake have been directed to use alternate sources of water for drinking and cooking because of concerns their filtering may be inadequate to handle the concentrations of toxins in the lake.

You can reach Staff Writer Mary Callahan at 707-521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.